This invention is directed to a holographic optical element directional diffusing screen by which monochromatic light striking the screen is diffused and directed by the screen. The screen may be a reflection or refraction holographic optical element. The exiting light is selectively distributed within a designated exit cone. The exit cone may have a central ray which is directed significantly away from the direction of impingement of monochromatic light on the rear surface.
The performance of a display for viewing by an observer is limited by the brightness and resolution of the image as perceived by a person in the viewing zone. In most cases the brightness of the image in a particular portion of a screen varies with the viewer's position. The screen is not at the same brightness over the whole area, and the distribution of the light is dependent upon the viewer's position within the viewing zone. This type of distribution of light on the screen makes it difficult for the viewer to observe all parts of the viewing screen and extract information therefrom with reliabity. Therefore, in many cases a screen of uniform brightness is desirable. In other cases a selected variation of brightness over the viewing area or pupil may be desired. Furthermore, it is desirable to be able to define a viewing pupil through which the majority of the light from the display screen is delivered. When the light can be delivered through a defined viewing pupil, with little scattering elsewhere, then brightness is controlled and enhanced because little light is wasted to scattering.
These purposes can be achieved by a display screen which is made by holographic exposure techniques which provide a diffusing screen of uniquely different characteristics as compared to a conventional ground glass diffusing screen. With such a screen the light leaving the screen is selectively distributed within a designated exit cone which may deviate significantly in angle from the direction of light impingement onto the screen. The directional diffusing screen so constructed is in essence a hologram that reconstructs a real image of a diffuse disc when illuminated by the appropriate reference beam. The parts of the hologram which are illuminated by the reference beam will themselves appear bright when viewed from the viewing pupil.
Prior work has been done on applications of holographic optical elements as viewing screens. For example see the article by Dietrich Meyerhofer in Applied Optics, Volume 12, Number 9, Sept. 9, 1973, at pages 2180 to 2184 entitled "Holographic and Interferometric Viewing Screens." Reference should also be made to American Journal of Physics 37:748 and Journal of the Optical Society of America 60:1635.